Oxygen Levels at Altitude

Oxygen Levels at Altitude

 

At high altitude, barometric pressure may be significantly lower than at sea-level. The result? Oxygen molecules are spread further apart, lowering the oxygen content of each breath one takes. Because of the reduced availability of oxygen in the air, blood oxygen levels decrease, and the body struggles to efficiently deliver oxygen to tissues, muscles and the brain.

We all live underneath a huge ocean of air that is several miles deep: the atmosphere. The pressure on our bodies is about the same as ten metres of sea water pressing down on us all the time. At sea level, because air is compressible, the weight of all that air above us compresses the air around us, making it denser. As you go up in elevation (while mountaineering, for example), the air becomes less compressed and is therefore thinner.

The important effect of this decrease in pressure is this: in a given volume of air, there are fewer molecules present. This is really just another way of saying that the pressure is lower (this is called Boyle’s law). The percentage of those molecules that are oxygen is exactly the same: 21% (20.9% actually). The problem is that there are fewer molecules of everything present, including oxygen.

Although the percentage of oxygen in the atmosphere is the same, the “thinner air” means there is less oxygen to breathe. Try using the Barometric Pressure Calculator to see how air pressure changes at high altitudes as well as how much less oxygen is available at any altitude.

The body makes a wide range of physiological changes in order to cope better with the lack of oxygen at high altitude. This process is called acclimatization. If you don’t acclimate properly, you greatly increase your chance of developing AMS (Acute Mountain Sickness), or even worse, HAPE (High Altitude Pulmonary Edema) or even HACE (High Altitude Cerebral Edema).

Why is There Less Oxygen at High Altitude?

What Happens to Your Body at High AltitudeAlthough the percentage of oxygen in inspired air is constant at different altitudes, the fall in atmospheric pressure at higher altitude decreases the partial pressure of inspired oxygen and hence the driving pressure for gas exchange in the lungs. An ocean of air is present up to 9-10 000 m, where the troposphere ends and the stratosphere begins.

The weight of air above us is responsible for the atmospheric pressure, which is normally about 100 kPa at sea level. This atmospheric pressure is the sum of the partial pressures of the constituent gases, oxygen and nitrogen, and also the partial pressure of water vapor (6.3 kPa at 37°C). As oxygen is 21% of dry air, the inspired oxygen pressure is 0.21×(100−6.3)=19.6 kPa at sea level.

Atmospheric pressure and inspired oxygen pressure fall roughly linearly with altitude to be 50% of the sea level value at 5500 m and only 30% of the sea level value at 8900 m (the height of the summit of Everest). A fall in inspired oxygen pressure reduces the driving pressure for gas exchange in the lungs and in turn produces a cascade of effects right down to the level of the mitochondria, the final destination of the oxygen.

Oxygen Levels By Altitude

Use the table below to see how the effective amount of oxygen in the air varies at different altitudes. Although air contains 20.9% oxygen at all altitudes, lower air pressure at high altitude makes it feel like there is a lower percentage of oxygen. The chart is based on the ideal gas law equation for pressure versus altitude (Barometric Formula), assuming a constant atmospheric temperature of 32 degrees Fahrenheit (0°C), and 1 atmosphere pressure at sea level.

Altitude (ft) Altitude (m) Effective O2 % Altitude Category Example
0 ft 0 m 20.9 % Low Altitude Sea Level
1000 ft 305 m 20.1 % Low Altitude  
2000 ft 610 m 19.4 % Low Altitude  
3000 ft 914 m 18.6 % Moderate Altitude  
4000 ft 1219 m 17.9 % Moderate Altitude  
5000 ft 1524 m 17.3 % Moderate Altitude Boulder, CO (5328 ft)
6000 ft 1829 m 16.6 % Moderate Altitude Mt. Washington (6288 ft)
7000 ft 2134 m 16.0 % Moderate Altitude  
8000 ft 2438 m 15.4 % High Altitude Aspen, CO (8000 ft)
9000 ft 2743 m 14.8 % high Altitude  
10,000 ft 3048 m 14.3 % High Altitude  
11,000 ft 3353 m 13.7 % High Altitude Mt. Phillips (11,711 ft)
12,000 ft 3658 m 13.2 % High Altitude Mt. Baldy (12,441 ft)
13,000 ft 3962 m 12.7 % Very High Altitude  
14,000 ft 4267 m 12.3 % Very High Altitude Pikes Peak (14,115 ft)
15,000 ft 4572 m 11.8 % Very High Altitude  
16,000 ft 4877 m 11.4 % Very High Altitude Mont Blanc (15, 781 ft)
17,000 ft 5182 m 11.0 % Very High Altitude  
18,000 ft 5486 m 10.5 % Extreme High Altitude  
19,000 ft 5791 m 10.1 % Extreme High Altitude Kilimanjaro (19,341 ft)
20,000 ft 6096 m 9.7 % Extreme High Altitude Denali (20,308 ft)
21,000 ft 6401 m 9.4 % Extreme High Altitude  
22,000 ft 6706 m 9.0 % Extreme High Altitude Aconcagua (22,841 ft)
23,000 ft 7010 m 8.7 % Extreme High Altitude  
24,000 ft 7315 m 8.4 % Extreme High Altitude  
25,000 ft 7620 m 8.1 % Extreme High Altitude  
26,000 ft 7925 m 7.8 % Ultra High Altitude  
27,000 ft 8230 m 7.5 % Ultra High Altitude  
28,000 ft 8534 m 7.2 % Ultra High Altitude K2 (28,251 ft)
29,000 ft 8839 m 6.9 % Ultra High Altitude Mt. Everest (29,029 ft)
Sources:

BMJ. 1998 Oct 17; 317(7165): 1063–1066.
doi: 10.1136/bmj.317.7165.1063
PMCID: PMC1114067
PMID: 9774298
ABC of oxygen

USGS Map Point Elevation Query Service
https://nationalmap.gov/epqs/

Source of Effective Oxygen %:
The answers given by the Barometric Formula equation.